The materials used in food packages are often plastic or cardboard-based materials with plastic or metal layers to add barrier properties. Food-containing packages normally have a structure with a bottom part containing the packed food and a cover part joined to the bottom part. The cover of the package often consists of two separate parts, one part intended for sealing the package tightly and the other part merely intended for resealing the package. The cover part for sealing the package tightly is normally film-like and it is removed completely when the package is opened, and after this, a separate plastic cover is used for resealing the package.
The film-like cover parts used for sealing the package are normally made of plastic, metal, or a cardboard-based material with plastic or metal layers for adding barrier properties, because a food package must, on one hand, be tightly sealable without affecting the general barrier properties of the package, but on the other hand, the package should be openable with relative ease later on.
For example publication WO 03/033258 discloses a heat-sealable package cover formed of a coated fibre material. Such a heat-sealable cardboard-based cover provides good sealing properties, and the cover is easily detachable from the package by tearing off. Such covers are normally used in food packages which are intended to be emptied at one time, but they are also used in connection with larger package sizes, and in this case, such a cardboard-based cover does not enable the resealing of the package. Consequently, a separate plastic cover must be used for resealing.
It is expensive to use such two-piece covers described above, because it increases, among other things, the need for materials, and such a cover structure is normally used in non-returnable packages. Another problem with separate plastic covers is how to keep them attached to the package during transportation. Thus, a simple and inexpensive cover structure which would enable both the tight sealing and resealing of the cover would be desirable. Furthermore, the sealing of food packages should be easy to include in the industrial food packaging process.